We ran this drill at the end of my last tactical shotgun class and it was a blast. We did it with 5 targets so during your run you would engage multiple targets depending on how many shells you were up to.
S-shooter
T-target
S1 engages T1
S2 -T1
S3 -T1
S4 -T1
S5 -T1
S1 -T1 & T2
S2 -T1 & T2
And so forth until
S5 engages T1, T2, T3, T4, & T5